As rare coin dealers, this is a question that often comes up from collectors working on their Lincoln Wheat Cent collection. Some coin collectors are looking for the 1922 from Philadelphia and the 1922 from the Denver mint. Let me put this discussion to bed quickly. The 1922 Lincoln Wheat Cent was only minted at the Denver facility. As such, there are 1922-D Lincoln Wheat Cents where the D is clearly visible and some where the 'D' has disappeared.
What happened to the 'D'? Apparently quality control wasn't as rigorous in the Roaring Twenties as it is today. The mint over used the dies and they began to wear out and the 'D' portion of the die got filled up with grease and gunk. Keep in mind that while the mint mark is raised on Lincoln Wheat Cents, it is incuse of the die. Dies are the inverse of the image to be struck. With a compacted and filled 'D' on an already worn out die, the mint mark failed to strike up properly.
But, the confusion doesn't end there. There are both 'No D' and 'Weak D' attributions in the rare coin market. The good news is that any numismatic item rare enough to be valuable is also rare enough that certain "fingerprints" exist. There are only three pairs of dies (obverse and reverse) that created the 1922 plain cent. They are cleverly labeled as Die Pair 1,2, & 3. Only Die Pair 2 is the 'No D' variety and brings all the money. I won't go into the diagnostics in this article. As of this writing 1922 No D Lincoln Wheat Cents quickly reach $1,000 or more. Die Pairs 1 & 3 are the 'Weak D' attribution or the poor man's No D and generally only bring a few hundred dollars in nice condition.
We buy these coins over the counter raw, but we're full time online coin dealers. If you're a collector, don't do it. 1922 No D cents are heavily counterfeited. Insist on buying them in PCGS, NGC, or ANACS certified holders. Coin collecting is a great hobby and this coin is a particular rarity among collectors.